Media Fingerprinting and Identification System

ABSTRACT

The overall architecture and details of a scalable video fingerprinting and identification system that is robust with respect to many classes of video distortions is described. In this system, a fingerprint for a piece of multimedia content is composed of a number of compact signatures, along with traversal hash signatures and associated metadata. Numerical descriptors are generated for features found in a multimedia clip, signatures are generated from these descriptors, and a reference signature database is constructed from these signatures. Query signatures are also generated for a query multimedia clip. These query signatures are searched against the reference database using a fast similarity search procedure, to produce a candidate list of matching signatures. This candidate list is further analyzed to find the most likely reference matches. Signature correlation is performed between the likely reference matches and the query clip to improve detection accuracy.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/856,670 entitled “A Highly Scalable, Accurateand Distortion-Robust Media Fingerprinting and Identification System”filed on Jun. 10, 2009 which is hereby incorporated by reference in itsentirety.

CROSS REFERENCE TO RELATED APPLICATIONS

U.S. application Ser. No. 12/141,163 filed on Jun. 18, 2008 entitled“Methods and Apparatus for Providing a Scalable Identification ofDigital Video Sequences”, U.S. application Ser. No. 12/141,337 filed onJun. 18, 2008 entitled “Method and Apparatus for Multi-dimensionalContent Search and Video Identification”, U.S. application Ser. No.12/491,896 filed on Jun. 25, 2009 entitled “Digital Video FingerprintingBased on Resultant Weighted Gradient Orientation Computation”, U.S.application Ser. No. 12/612,729 filed Nov. 5, 2009 entitled “DigitalVideo Content Fingerprinting Based on Scale Invariant Interest RegionDetection with an Array of Anisotropic Filters”, U.S. ProvisionalApplication Ser. No. 61/181,806 filed on May 28, 2009 entitled“Multi-Media Content Identification Using Multi-Level Content SignatureCorrelation and Fast Similarity Search”, and U.S. ProvisionalApplication Ser. No. 61/266,668 filed on Dec. 4, 2009 entitled “DigitalVideo Content Fingerprinting Using Image Pixel Intensity and ColorInformation” have the same assignee as the present application, arerelated applications, and are hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates generally to improvements in representingand identifying multimedia information. More particularly, the presentinvention addresses a scalable and accurate audio and videofingerprinting and identification system that is robust to many classesof audio and video distortions.

GENERAL BACKGROUND OF THE INVENTION

Media applications which include video and audio database management,database browsing and identification are undergoing explosive growth andare expected to continue to grow. To address this growth, there is aneed for a comprehensive solution related to the problem of creating avideo sequence database and identifying, within such a database, aparticular video sequence or sequences that are tolerant of mediacontent distortions. Multiple applications include video databasemining, copyright content detection for video hosting web-sites,contextual advertising placement, and broadcast monitoring of videoprogramming and advertisements.

Multimedia fingerprinting refers to the ability to generate associatedidentifying data, referred to as a fingerprint, from the multimediaimage, audio and video content. A fingerprint ideally has severalproperties. First, the fingerprint should be much smaller than theoriginal data. Second, the fingerprint should be designed such that itcan be searched for in a large database of fingerprints. Third, theoriginal multimedia content should not be able to be reconstructed fromthe fingerprint. Fourth, for multimedia content that is a distortedversion of another multimedia content, fingerprints of the original anddistorted versions should be similar. Examples of some common multimediadistortions include, selecting a clip of video content temporally,cropping the image data, re-encoding the image or audio data to a lowerbit-rate, changing a frame rate of the video or audio content,re-recording the multimedia data via some analog medium such as acamcorder in a movie theatre, and changing the aspect ratio of the imagecontent. A fingerprint with the fourth property is deemed to be robustagainst such distortions.

Such a system of fingerprinting and search is preferable to othermethods of content identification. For example, multimedia watermarkingchanges the multimedia content by inserting watermark data. Unlikemultimedia watermarking, fingerprinting does not change the content.Fingerprinting is, however, a very challenging problem.

Increasing demand for such fingerprinting and search solutions, whichinclude standard definition (SD) and high definition (HD) formats ofvideo, requires increasing sophistication, flexibility, and performancein the supporting algorithms and hardware. The sophistication,flexibility, and performance that are desired exceed the capabilities ofcurrent generations of software based solutions, in many cases, by anorder of magnitude.

SUMMARY OF THE INVENTION

In one or more of its several aspects, the present invention recognizesand addresses problems such as those described above. The presentinvention describes a system that is highly scalable to very largemultimedia databases, highly accurate in finding a closest matchingmultimedia clip and not misidentifying a wrong clip, and highly robustto many types of distortion. It is also very efficient in terms of thecomputer hardware resources necessary to fingerprint and search. To suchends, an embodiment of the invention addresses a method of searchingreference multimedia signatures that represent audio and video contentof reference multimedia clips for a corresponding query multimedia clip.An N dimension signature based on an N dimension descriptor and atraversal hash signature based on a combination of dimensions of the Ndimension descriptor are generated for each feature identified in a setof reference multimedia clips. Each N dimension signature and a link toa corresponding reference multimedia clip are stored at a location in areference signature database addressable by the traversal hashsignature. An N dimension query signature based on an N dimension querydescriptor and a traversal hash query signature based on a combinationof dimensions of the N dimension query descriptor are generated for aquery multimedia clip. The reference signature database is searchedusing the traversal hash query signature to find similar signatures thatare within a specified signature distance, wherein the similar referencemultimedia clips are aggregated in a candidate list of closely matchingsignatures that correspond to similar reference multimedia clips.

Another embodiment of the invention addresses a method of formingaccurate descriptors of features in multimedia clips. A scale and alocation of the center parameters are determined for each feature of aplurality of features selected from a frame from a multimedia clip. A Bxby By grid is created having Bx*By blocks located at the center of eachfeature and each block having a Cx by Cy pixel grid. For each block,functions of multiple intensity differences are computed between eachpixel in a block's Cx by Cy pixel grid and another pixel for a specifiedpixel offset that is greater than 1 and based on the scale and locationof center parameters. A first N dimension descriptor is generated as aconcatenation of the functions for each block of the Bx*By blocks. Thescale and location of center parameters are individually adjusted toaccount for an expected error margin due to image distortions togenerate at least a second N dimension descriptor as a concatenation ofthe functions for each block of the Bx*By blocks as adjusted accordingto the expected error margin.

Another embodiment of the invention addresses a computer readable mediumstoring a computer program which causes a computer system to perform amethod of searching reference multimedia signatures that represent audioand video content of reference multimedia clips for a correspondingquery multimedia clip. An N dimension signature based on an N dimensiondescriptor and a traversal hash signature based on a combination ofdimensions of the N dimension descriptor are generated for each featureidentified in a set of reference multimedia clips. Each N dimensionsignature and a link to a corresponding reference multimedia clip arestored at a location in a reference signature database addressable bythe traversal hash signature. An N dimension query signature based on anN dimension query descriptor and a traversal hash query signature basedon a combination of dimensions of the N dimension query descriptor aregenerated for a query multimedia clip. The reference signature databaseis searched using the traversal hash query signature to find similarsignatures that are within a specified signature distance, wherein thesimilar reference multimedia clips are aggregated in a candidate list ofclosely matching signatures that correspond to similar referencemultimedia clips.

A further embodiment of the invention addresses method for creating amedia database. A plurality of signatures are generated from descriptorsof selected media content. A signature is selected from the plurality ofsignatures based on a distribution of bit values in the selectedsignature and on values of a corresponding descriptor. A traversal hashsignature is generated from the corresponding descriptor. The selectedsignature is stored in a media database at a location within a set ofcontiguous locations based on the traversal hash signature used as aprimary index.

These and other features, aspects, techniques and advantages of thepresent invention will be apparent to those skilled in the art from thefollowing detailed description, taken together with the accompanyingdrawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for media content fingerprinting inaccordance with the present invention;

FIG. 2A illustrates a reference media database generation process inaccordance with the present invention;

FIG. 2B illustrates a query fingerprint generation process in accordancewith the present invention;

FIG. 2C illustrates a similarity search process in accordance with thepresent invention;

FIG. 2D illustrates a candidate video filtering process in accordancewith the present invention;

FIG. 2E illustrates a signature correlation process in accordance withthe present invention;

FIG. 3 illustrates a multimedia fingerprinting process in accordancewith the present invention;

FIG. 4 illustrates a scale-space feature in accordance with the presentinvention;

FIG. 5A illustrates an exemplary rectangular descriptor box andcoordinates for use in generating a global descriptor in accordance withthe present invention;

FIG. 5B illustrates an exemplary resampled rectangular descriptor boxfor use in generating a global descriptor in accordance with anembodiment of the present invention;

FIG. 6 illustrates aspects of a global descriptor gradient computationprocess in accordance with the present invention;

FIG. 7 illustrates multiple robust descriptors in accordance with thepresent invention;

FIG. 8A illustrates an exemplary signature generation process inaccordance with the present invention;

FIG. 8B illustrates a threshold scale used in a procedure to augment atraversal hash signature in accordance with the present invention;

FIG. 8C illustrates a signature selection procedure which selectssignatures for a particular frame of video in accordance with thepresent invention;

FIG. 8D illustrates a procedure to generate the temporal robustnessestimate in accordance with the present invention;

FIG. 9A illustrates an exemplary signature database organized by aprimary hash signature index in accordance with the present invention;

FIG. 9B illustrates an exemplary signature database organized bysecondary index in accordance with the present invention;

FIG. 10 illustrates an exemplary similarity search process in accordancewith the present invention;

FIG. 11 illustrates a query multimedia clip partitioned into smallersegments in accordance with the present invention;

FIG. 12 illustrates an exemplary listing of top matches in accordancewith the present invention; and

FIG. 13 illustrates a signature correlation process in accordance withthe present invention.

DETAILED DESCRIPTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which several embodiments of the inventionare shown. This invention may, however, be embodied in various forms andshould not be construed as being limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the scope of theinvention to those skilled in the art.

It will be appreciated that the present disclosure may be embodied asmethods, systems, or computer program products. Accordingly, the presentinventive concepts disclosed herein may take the form of a hardwareembodiment, a software embodiment or an embodiment combining softwareand hardware aspects. Furthermore, the present inventive conceptsdisclosed herein may take the form of a computer program product on acomputer-readable storage medium having computer-usable program codeembodied in the medium. Any suitable computer readable medium may beutilized including hard disks, CD-ROMs, optical storage devices, flashmemories, or magnetic storage devices.

Computer program code or software programs that are operated upon or forcarrying out operations according to the teachings of the invention maybe written in a high level programming language such as C, C++, JAVA®,Smalltalk, JavaScript®, Visual Basic®, TSQL, Perl, use of .NET™Framework, Visual Studio® or in various other programming languages.Software programs may also be written directly in a native assemblerlanguage for a target processor. A native assembler program usesinstruction mnemonic representations of machine level binaryinstructions. Program code or computer readable medium as used hereinrefers to code whose format is understandable by a processor. Softwareembodiments of the disclosure do not depend upon their implementationwith a particular programming language.

The methods described in connection with the embodiments disclosedherein may be embodied directly in hardware, in a software moduleexecuted by a processor, or in a combination of the two. A softwaremodule may reside in RAM memory, flash memory, ROM memory, EPROM memory,EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or anyother form of storage medium known in the art. A computer-readablestorage medium may be coupled to the processor through local connectionssuch that the processor can read information from, and write informationto, the storage medium or through network connections such that theprocessor can download information from or upload information to thestorage medium. In the alternative, the storage medium may be integralto the processor.

A fingerprint system should be robust to distortions, while allowing forhigh matching accuracy and low false positive rate. A fingerprintingsystem should have fast search speed, especially when searching over alarge database of content. The fingerprinting system should be able toreturn a match for a relatively short clip of query content. Thefingerprinting system should also be able to generate fingerprintsrapidly when generating a reference database or query fingerprints to besearched. These goals tend to compete with each other. For instance,high search speed and support for short query clips can come at the costof accuracy or robustness to distortion. Generally, accuracy trade-offsagainst robustness to distortion.

Due to the large size of such databases and the density of media files,high performance, accurate media identification and search technologiesare needed. Furthermore, robust video content identification,management, and copyright protection should be resistant to intentionalor unintentional video content change or distortion within the limits ofparameters, such as, reasonable view-ability. The overall design shouldbe scalable to efficiently handle very large databases of videos and anarbitrary length of a query sequence.

To provide for such needs, FIG. 1 illustrates a system 100 for bothmedia fingerprinting and identification in accordance with an embodimentof the present invention. The system 100 includes user sites 102 and103, a server 106, a video database 108, and a video fingerprinting andvideo identification system 112 implemented as a program and operated,for example, by user site 102. A network 104, such as the Internet, awireless network, or a private network, connects sites 102 and 103 andserver 106. Each of the user sites, 102 and 103 and server 106 mayinclude a processor complex having one or more processors, havinginternal program storage and local user controls such as a monitor, akeyboard, a mouse, a printer, and may include other input or outputdevices, such as an external file storage device and communicationinterfaces.

The user site 102 may comprise, for example, a personal computer, alaptop computer, or the like equipped with programs and interfaces tosupport data input and output and video fingerprinting and searchmonitoring that may be implemented both automatically and manually. Theuser site 102, for example, may store programs, such as the videofingerprinting and search system 112 program implementation of a contentbased video identification process of the present invention or haveaccess to such programs through electronic media, such as may bedownloaded over the Internet from an external server, accessed through auniversal serial bus (USB) port from flash memory, accessed from diskmedia of various types, or the like. The system 100 may also suitablyinclude more servers and user sites than shown in FIG. 1. Also, multipleuser sites each operating an instantiated copy or version of the videofingerprinting and search system 112 may be connected directly to theserver 106 while other user sites may be indirectly connected to it overthe network 104.

User sites 102 and 103 may generate user video content which is uploadedover the Internet 104 to a server 106 for storage in a video database108. The user sites 102 and 103, for example, may also operate a videofingerprinting and video identification system 112 to generatefingerprints and search for video content in the video database 108. Thevideo fingerprinting and video identification system 112 in FIG. 1 isscalable and utilizes highly accurate video fingerprinting andidentification technology that checks unknown video content against adatabase of previously fingerprinted video content, which is consideredan accurate or “golden” database. The video fingerprinting and videoidentification system 112 is different from commonly deployed systems inthat it extracts features from the video itself. The videofingerprinting and video identification system 112 allows the server 106to configure a “golden” database specific to its business requirements.The user site 102 that is configured to connect with the network 104,uses the video fingerprinting and search system 112 to compare videostreams against a database of signatures in the video database 108, thathas been previously generated.

As used herein, a video fingerprint is a calculated numerical value,alternatively termed a signature, which represents a detected feature ofa video frame. In one approach, a feature detection procedure uses a twopass scale space analysis as described in U.S. application Ser. No.12/612,729 filed Nov. 5, 2009 entitled “Digital Video ContentFingerprinting Based on Scale Invariant Interest Region Detection withan Array of Anisotropic Filters”, FIGS. 3, 4, 5, 6A, 6B, 7A, 7B, and 8and pages 16-26 and 34-36. The first pass uses Laplacian-of-Gaussiansecond order partial derivative bi-level filters to generate interestregions. These interest regions are further refined in the second pass,using anisotropic filters of sampled elliptic-shaped Gaussian secondorder partial derivative filters. The detected features, found byinterpolating local maxima values, are characterized by the followingset of parameters (s_(x), s_(y), x, y, peak polarity), where s_(x),s_(y) denote the scale in the x and y directions, (x,y) denote the pixellocation, and peak polarity denotes whether the feature is a dark orlight relative to the image background. For each feature that isdetected in a video frame, one or more descriptors are generatedaccording to a global descriptor procedure described below. For eachglobal descriptor, a pair of signatures, a primary signature and atraversal hash signature, are generated as described in more detailbelow. A collection of signatures drawn from a single video frame withmultiple features generates a compact video frame representation, whilea collection of signatures associated with each video frame in a videosequence with multiple frames forms the video sequence representation.

The user site 102 has access to the video database 108. The videodatabase 108 may store the video archives, as well as all data relatedto various video content stored in the video database 108, and aplurality of video fingerprints that have been adapted for use asdescribed herein and in accordance with the present invention. It isnoted that depending on the size of an installation, the functions ofthe video fingerprinting and search system 112 and the capacity andmanagement of the video database 108 may be combined in a singleprocessor system, such as user site 102 or server 106, running separateprogram threads for each function.

One embodiment of the present invention addresses the problem ofmatching a query multimedia clip to a reference multimedia clip from alarge collection of reference multimedia clips. The multimedia clip andthe reference multimedia clips are processed to generate a compact,searchable set of fingerprints and a multimedia fingerprint database isgenerated to allow for fast, accurate searching using a queryfingerprint to search this multimedia fingerprint database.

At the base of the problem of matching a video clip to a video from alarge collection of videos, is the problem of reliably matching twodigital images when one of them has undergone some form of distortion,such as geometric and optical distortions. Thus, the problem may berestated as what techniques are required to reliably match a distortedquery video frame, chosen from a query video clip, to a correspondingundistorted, original video frame residing in a database of originalvideo material. Since the database of original video material may bevery large, many video frames in this database could exhibit substantialstatistical similarity. Furthermore, many features within a single videoframe may also show statistical similarity in both a visual or opticaland algorithmic or geometric sense. This fact makes it difficult toclearly distinguish between many possible feature descriptor matchesresulting from a database browsing or search activity when a non-exact,similarity measure is used. Hence, a precise, discriminative, and robustvideo frame feature characterization is desirable having a featuredetector and identifier that are well localized in terms of its locationspatially and its spatial extent or scale.

This invention describes a system that is readily scalable to very largemultimedia databases, accurate in finding a correct clip and notmisidentifying a wrong clip, and robust to many types of distortion. Afingerprinting and search system is used, where the fingerprint for apiece of multimedia content is composed of a number of compactsignatures, including traversal hash signatures and associated metadata.The compact signatures and traversal hash signatures are constructed tobe easily searchable when scaling to a large database of multimediafingerprints. The multimedia content is also represented by manysignatures that relate to various aspects of the multimedia content thatare relatively independent from each other. Such an approach allows thesystem to be robust to distortion of the multimedia content even whenonly small portions of the multimedia content are available.

Multimedia, specifically audio and video content, may undergo severaldifferent types of distortions. For instance, audio distortions includere-encoding to different sample rates or audio quality. Videodistortions include cropping, stretching, re-encoding to a lowerquality, and image overlays. While these distortions change the digitalrepresentation, the multimedia is perceptually similar to a humanlistener or viewer. Robustness to these distortions refers to theproperty that content that is perceptually similar will generatefingerprints that have a small distance according to some distancemetric, such as Hamming distance for bit based signatures, and contentthat is perceptually distinct from one another will generatefingerprints that have a large distance, according to the same distancemetric. A search for perceptually similar content, hence, is transformedto a problem of searching for fingerprints that are a small distanceaway from the desired fingerprints.

The overall architecture and details of a highly scalable and accuratevideo fingerprinting and identification system is described that isrobust to many classes of video distortions. In this system, thefingerprint for a piece of multimedia content is composed of a number ofcompact signatures, along with traversal hash signatures and associatedmetadata. The compact signatures and traversal hash signatures areconstructed so as to be easily searchable when scaling to a very largedatabase of multimedia fingerprints. The multimedia content isrepresented by many signatures that relate to various aspects of themultimedia that are relatively independent from each other. This allowsthe system to be very robust under heavy distortion when only smallportions of the multimedia content are available.

The content based multimedia identification system consists of amultimedia fingerprinting process and a search process. In order toidentify an unknown multimedia clip, these two components are used asdescribed in more detail below.

FIG. 2A illustrates a reference media database generation process 200 inaccordance with the present invention. Reference multimedia clips 202 ₁,202 ₂, . . . 202 _(N) that are relevant to the application at hand areidentified. The clips 202 ₁, 202 ₂, . . . 202 _(N) refer to distinctpieces of multimedia content. For example, the clips could be from amovie and represent different temporal portions of the movie, includingaudio and video content, or they could be from different movies. Using avideo fingerprinting function 203 of the video fingerprinting and searchsystem 112, reference signatures 204 ₁, 204 ₂, . . . 204 _(N) aregenerated for the reference multimedia clips 202 ₁, 202 ₂, . . . 202_(N), respectively, along with hashing data and associated metadata.Different pieces of multimedia content may be fingerprintedindependently, leading to a parallelizable system. A clip of multimediacontent can be broken down into its temporal components, such as framesin the case of video, or temporal audio segments in case of audio. Evenmore parallelism is possible by independently fingerprinting at thetemporal component level, or frame level.

The set of reference signatures 204 ₁, 204 ₂, . . . 204 _(N) created inthe video fingerprinting function 203 is organized by databasegeneration function 206 into a reference database 208. This set ofreference signatures is indexed by the generated hashing data, describedin further detail below. The associated metadata, also described infurther detail below, is stored along with each reference signature. Aset of reference signatures may also be indexed in other ways, forinstance, by multimedia identifiers. A single multimedia identifierdenotes a distinct piece of multimedia content. For instance, themultimedia clips 202 ₁, 201 ₂, . . . 202 _(N) would each be representedby their own multimedia identifier.

FIG. 2B illustrates a query fingerprint generation process 220 inaccordance with the present invention. A user requests identification ofan unknown multimedia clip 222, including audio and video content, alsoreferred to herein as a query multimedia clip 222. The query multimediaclip 222 is processed by the video fingerprinting function 203 togenerate query signatures, hash data, and associated metadata, knowncollectively as a query fingerprint 224, for the unknown multimedia clip222.

FIG. 2C illustrates a similarity search process 230 in accordance withthe present invention. For each query signature in the query fingerprint224, a similarity search function 232 is initiated to find similarsignatures in the reference database 208. The hash data associated witheach query signature is used to restrict the similarity search function232 to a relatively small portion of the reference data, allowing thesimilarity search to be extremely fast even for large referencedatabases. Only reference signatures that are “similar” within adistance measure to the query signature are returned. These classifiedsimilar reference signatures are added to a candidate list 234, whichcontains identifying information regarding which reference multimediaclip 202 ₁, 202 ₂, . . . 202 _(N) the similar reference signaturebelongs.

FIG. 2D illustrates a candidate video filtering process 240 inaccordance with the present invention. The video filtering process 240analyzes the candidate list 234 for the most likely matches in thereference database 208. The candidate list 234 is sorted in topmultimedia clips function 242 to find the top most likely matchingmultimedia clips. The resulting data is stored in a list of top clips244. The list of top clips 244 includes a multimedia identifier for thesimilar reference multimedia clip. The matching multimedia clip might beonly for a specific portion of the multimedia clip, for instance, aparticular time segment, such as seconds 93 to 107 of a video sequence,or spatial locations, such as top left quadrant of the video frames. Thetemporal identification and spatial locations are also included in thelist of top clips.

FIG. 2E illustrates a signature correlation process 250 in accordancewith the present invention. The list of top clips 244 is selected forcorrelation. For each of the clips in the top clips list 244, a set ofsignatures is accessed from the reference database 208 given themultimedia identifier and any temporal and/or spatial information. Aquery could correspond to “all signatures for video number ABC from time10.4 seconds to 25.7 seconds in the bottom-right quadrant of the frame.”These signatures are not restricted have matching traversal hashsignatures. The set of query signatures is now compared against this setof reference signatures, using the signature correlation procedure 252.For each query signature, a score is derived based on the distances tothe closest signatures in the reference database signature subset, andthe number of matches. This could include, for instance, the averagedistance. These scores are combined, for example, the scores areaveraged, for the entire set of query signatures to give an overallscore for the database clip. Based on whether this score is over athreshold, the database clip is determined to be a true positive or afalse positive. The signature correlation procedure 252 is repeated forall clips in the top clips list 244, to produce a list of matchingreference videos, since there may be more than one, if similar contentis repeated in the database. FIG. 2A fingerprinting function 203 andFIG. 2B belong to the multimedia fingerprinting system, while FIG. 2Adatabase generation function 206 and FIGS. 2C-2E belong to the searchsystem.

FIG. 3 illustrates a multimedia fingerprinting process 300 in accordancewith the present invention. A video clip is decoded from a multimediasource, such as an motion picture experts group (MPEG) compressed audioand video stream, into individual frames and a subset of the frames areselected for processing. For each selected frame, features are detectedusing a scale-space feature detector. An N-dimensional descriptor isgenerated based on the image pixels corresponding to the scale andposition values from the feature descriptor. An M-dimensional signatureand K-dimensional traversal hash signature is generated from descriptor.

The multimedia fingerprinting process 300 encompasses the fingerprintingfunction 203 shown in FIG. 2A and the query fingerprint generationprocess 220 of FIG. 2B. As shown in FIG. 3, the multimediafingerprinting process 300 is comprised of a decoding step 304, aselecting frame step 306, a detecting audio and video features step 308,a describing the detected audio and video features step 310, agenerating signatures and hash data step 312, and a selecting signaturesstep 314.

The decoding step 304 comprises decoding a multimedia stream that may beaccessed in compressed form from a distributed source to produce raw orunprocessed image and audio data. This data, which is associated with anidentified playout timestamp, can be combined with other contiguous datato form a “frame” of data. For example, for image data, a frame could bea single image, or group of images, decoded from the multimedia stream.For audio data, a frame could be several milliseconds worth of decodedaudio.

The selecting frame step 306 comprises selecting a subset of allavailable frames for further content analysis. Image and audio dataexhibit a great deal of temporal coherence which makes it possible toselect a limited number of frames from the entire set of data and obtainsimilar accuracy as if all frames were used. The search system may alsobe more efficient when fewer signatures are used. The selecting framestep 306 uses an algorithm to select the subset of all available framesto represent the entire multimedia sequence, as described in more detailbelow.

The detecting audio and video features step 308 comprises furtheranalyzing each selected frame to find audio and video features thatshould be reproducible under an expected suite of distortions. Areproducible feature implies a high likelihood of having correspondingfeatures in the original content and the distorted content. For animage, this may involve doing scale-space analysis or using a blobdetector, to find an x and y location and an x and y scale of theidentified blobs in the image.

The describing the detected audio and video features step 310 comprisesgenerating a compact descriptor for each of the features detected in theprevious step. For image data, the compact descriptor, such as a vectorof real values, could be based on intensity and gradient values in aspatial area surrounding a detected blob. Generally, such a compactdescriptor has on the order of 64 or 128 components, based on spatialsubdivision and various image spatial and temporal derivatives ofzeroth, first and higher orders.

The generating signatures and hash data step 312 comprises, for eachgenerated descriptor of a particular feature of a frame of multimedia,generating an associated signature and a signature hash. The signaturesgenerated typically quantize a descriptor's real value and generate a1-bit, 2-bit, 4-bit, or 8-bit representation for each of thedescriptor's components. For a descriptor that is 64-dimensional, we canuse a single threshold value to generate a 64-bit signature. By summingmultiple descriptor dimensions, for instance, 4 dimensions, andthresholding the resulting vector, a 16-bit signature hash value can begenerated to associate with this signature.

The selecting signatures step 314 comprises selecting signatures thatmay have greater information content about the multimedia content. Forinstance, bit-signatures that have nearly all 0's or all 1's tend not tobe very unique to a particular multimedia content. Hence, signatureswith a count of 0's or 1's that fall below some threshold may beremoved. Signatures may also be rejected based on descriptorinformation, for the descriptor from which the signature is derived. Bysumming the descriptor values and rejecting those with a sum below athreshold, signatures are eliminated that have corresponding descriptorswith their energy concentrated in only a few dimensions.

The multimedia fingerprinting process 300 is described in more detailbeginning with the frame selection step 306. The frame selection step306 includes both static and dynamic selection of frames. In staticframe selection, frames are selected in a specified frame selectionperiod of N frames, where, for example, every N^(th) frame is selectedwithout particular regard to frame content. This approach is used forcertain applications where processing of every frame, would be difficultdue to computational load. In dynamic frame selection, frames areselected based on their audio and video content. Other aspects andfurther details regarding frame selection may be found in U.S.application Ser. No. 12/141,163 filed on Jun. 18, 2008 entitled “Methodsand Apparatus for Providing a Scalable Identification of Digital VideoSequences”, FIGS. 3 and 12 and pages 14-16, 25, and 26, and U.S.application Ser. No. 12/491,896 filed on Jun. 25, 2009 entitled “DigitalVideo Fingerprinting Based on Resultant Weighted Gradient OrientationComputation”, FIG. 8 and pages 20-22. For dynamic frame selection, aframe selection period is also specified. The frame selection algorithmand period affects the total processing time, the number of signaturesgenerated, as well as the accuracy of the results.

The detecting audio and video features step 308 involves finding someaspect of the multimedia content for the selected frame that should bereproducible under an expected suite of distortions. In accordance withan embodiment of the present invention, an advantageous approach forfinding reproducible image features is described. It is noted that thisapproach can also be adapted to finding reproducible audio features.Both image and audio features may also be described by a compactdescriptor.

Image features, such as blobs in an image, may be identified by locationand scale in an x and y direction by using a form of scale-spaceanalysis, also referred to herein as blob detection. Conceptually,scale-space analysis applies Gaussian smoothing filters at a particularscale to the original image, and computes a determinant of the Hessian,to produce a response image for that scale. By appropriately selectingpeaks from these response images, x and y locations and scales may beinferred.

Further details regarding scale-space analysis may be found in U.S.Application Ser. No. 12/612,729 filed Nov. 5, 2009 entitled “DigitalVideo Content Fingerprinting Based on Scale Invariant Interest RegionDetection with an Array of Anisotropic Filters”, FIGS. 3, 4, 5, 6A, 6B,7A, 7B, and 8 and pages 16-26 and 34-36. Other feature detectiontechniques may also be alternatively used in the fingerprinting systemof the present invention.

The multimedia fingerprinting process 300 continues with step 310 togenerate one or more feature descriptors. A rectangular region ofinterest in the image is computed based on the scale and location of thedetected feature. “Global gradient” values, a more general form ofgradient values, are computed in the x and y directions over the pixelsof the region of interest. The descriptor is computed according to anonlinear weighting of the global gradient values, producing a resultwhich is accumulated into +x, −x, +y, −y bins, for multiple regions inthe region of interest. The end result is an M-dimensional descriptor.

An example of a detected scale space feature is shown in FIG. 4. FIG. 4illustrates a scale-space feature 400 in accordance with the presentinvention. In FIG. 4, an image feature is represented by dashed outline409, which could be a contiguous area of near-constant colored pixelthat is, for instance, a human face, or part of a picture on a wall. Forthis image feature, a corresponding image blob 402 is located in animage space 404 by (centerx, centery) coordinates 406 and with aparticular s_(x) 407 and s_(y) 408 scale reference. A scale-spacefeature has a few others attributes that describe it relative to thesurrounding image pixels. For example, a peak polarity value is a binaryvalue indicating whether a feature is a dark blob on a light background,or a light blob on a dark background. A peak strength value represents ameasure of how much a blob stands out from its background. For example,a light gray blob on a dark gray background has a lower peak strengthvalue, as compared to a black blob on a white background which has ahigher peak strength value.

The step 310 for describing the detected audio and video featuresincludes generating a global descriptor. FIG. 5A illustrates anexemplary rectangular descriptor box and coordinates 500 for use ingenerating a global descriptor 500 in accordance with the presentinvention. Other aspects and further details regarding descriptors, suchas a phase descriptor, may be found in U.S. application Ser. No.12/612,729 filed Nov. 5, 2009 entitled “Digital Video ContentFingerprinting Based on Scale Invariant Interest Region Detection withan Array of Anisotropic Filters”, FIGS. 9, 10, and 11 and pages 26-28,38, and 39.

The detecting audio and video features step 308 generates four outputvalues which include (centerx, centery) coordinates 406, s_(x) 407, ands_(y) 408 which represent the center of an image blob 504 and a spatialextent of the blob. A rectangular box 506 is placed on the image blobwith center at the blob (centerx, centery) coordinates 406 and spatialextent (Ns_(x) 502, Ms_(y) 503), where N and M are multiplicationfactors that determines the size of the neighborhood around the blobthat the descriptor considers. This rectangular box 506 is subdividedinto a Bx by By grid, with, for example, Bx=4 and By=4 providing a 4×4grid to create Bx*By=16 blocks

FIG. 5B illustrates an exemplary resampled rectangular descriptor box515 for use in generating a global descriptor in accordance with anembodiment of the present invention. In FIG. 5B, an exemplaryrectangular descriptor box 522 corresponding to 506 in FIG. 5A, iscentered at an interest point 530, with Bx*By=16 sub-regions, where Bxand By both equal 4. In more detail and with specified featureparameters (centerx, centery, s_(x), s_(y)) a rectangular descriptor box522 is placed on the image with center at (x, y) interest point 530 andwith spatial extent of Ns_(x), Ms_(y), where N and M are multiplicationfactors which determine the size of the neighborhood around the interestpoint under consideration. The rectangular descriptor box 522 isresampled to generate a resampled grid 524 with (Bx*By)(Rx*Ry), where Rxand Ry equal 5 in this example, pixel regions, such as region 528 forblock 526. For each block, such as the block 526, a Rx*Ry pixel grid 521with associated Rx*Ry re-sampled pixel values in region 528 is computedas follows. First, lines that define the Rx*Ry sub-grid are determined,where these lines are given with sub-pixel accuracy and are notnecessarily aligned with the pixel boundaries 521. In the x direction,the lines evenly subdivide the block 521 horizontally and are namedLnx0, Lnx1, . . . Lnx(Rx). Likewise, in the y direction, the linesevenly subdivide the block 521 vertically and are named LnyO, Lnyl, . .. Lny(Ry). The locations of the pixels contained within rectangle 521are shown as dots. The pixels contained within each of the Rx*Rysub-grid areas are combined to produce a resampled pixel. This is doneby summing the intensity values and dividing by the number of pixels,within each of the Rx*Ry sub-grid areas, yielding Rx*Ry pixel intensityvalues. Note that there may be different number of pixels for some orall of the Rx*Ry sub-grid areas. There may also be no pixels in a Rx*Rysub-grid area, in which case the resampled intensity value is taken aszero.

This Rx*Ry pixel sub-region, computed for each of the Bx*By blocks ofthe rectangular box drawn around an interest point, is subsequently usedto generate a descriptor and, based on this descriptor, a signature forthat interest point.

FIG. 6 illustrates aspects of a global descriptor gradient computationprocess 600 in accordance with the present invention. Each Rx*Ry pixelgrid that is computed for each of the Bx*By blocks is used to generate 4values. These values are “global gradient” values computed according tothe following procedure. For each pixel in an Rx*Ry grid, thedifferences from a pixel to several other pixels are computed. FIG. 6illustrates the pixels on which these differences are taken, forexample, from the reference pixel 602, to the target pixels 603, 604,605 and 606. These target pixels may be chosen arbitrarily. In thisexample, they have been chosen to be to the right 2 pixels and 4 pixels,as well as down 2 pixels and 4 pixels. Other pixel positions are alsopossible. Based on these pixels, four difference values are computed,including difference A 607 computed between pixel 603 and pixel 602,difference B 608 computed between pixel 604 and pixel 602, difference C609 computed between pixel 605 and pixel 602, difference D 610 computedbetween pixel 606 and pixel 602. Pixels, such as pixels 604 and 606,might not exist in a particular Rx*Ry pixel grid for pixels near to theright or bottom borders, so such pixels will access the appropriatepixel in a neighboring Rx*Ry pixels grid. If a bordering Rx*Ry pixelgrid does not exist, an additional block or blocks might need to becomputed along the right and bottom borders of the rectangular box 506.This means, for instance, that instead of a Bx by By grid computed, a(Bx+1) by (By+1) grid is computed, with reference pixels within theoriginal Bx by By grid and target pixels potentially accessing thelarger (Bx+1) by (By +1) grid.

These difference values are combined into one or more values, using aweighted average of the values. For example, an “x-gradient” is computedas the weighted sum of A and B,

xgrad=w ₁ *A+w ₂ *B,

and, similarly, a “y-gradient” is computed as the weighted sum of C andD,

ygrad=w ₃ *C+w ₄ *D.

The weights, w₁ to w₄, may depend on many factors, but in oneembodiment, they depend on an agreement of signs of the differencevalues A and B, and C and D. For instance, if A and B are of the samesign, then w₁=1.1 and w₂=0.2, whereas if A and B are of differentvalues, then w₁=1.2 and w₂=0.0. Different weights can be used for the xand y directions. The weights can also depend on decision factors otherthan the agreement of signs.

For example, an xgrad value and a ygrad value may be computed for eachof the pixels associated with the Rx*Ry pixel grid 521. From this set ofvalues, four sums are computed as global gradient values:

1. −1*sum of xgrad for each pixel, where xgrad is negative

2. sum of xgrad for each pixel, where xgrad is positive

3. −1*sum of ygrad for each pixel, where ygrad is negative

4. sum of ygrad for each pixel, where ygrad is positive

In total, a descriptor for the rectangular box 506 consists of 4*Bx*Byvalues that is a concatenation of the 4 “global gradient” values foreach of the Bx*By blocks covered by the descriptor. By design, thesevalues are all positive. For example, for global gradient values 1 and 3in the above list, the sums are of negative values, to produce anegative sum, which is made positive by multiplying by −1 and for globalgradient values 2 and 4, the sums are of positive values, producing apositive sum.

Some image distortions may cause the feature detection algorithm to haveerrors in the detected scale values or location. If the error is largeenough, this might result in differences in the descriptor, andsubsequently in the signature, that are too large for matching to workcorrectly. To improve the chances of matching, that is, make thefingerprint more robust with respect to certain image distortions, thefollowing embodiment of generating multiple different descriptors basedon the same detected feature may be used. These multiple descriptors maybe generated, for example, by perturbing the scale value and/or thelocation value. For example, the scalex multiplication factor, such as Nof FIG. 5A, or the scaley multiplication factor, such as M of FIG. 5A,or both are adjusted to account for an expected error margin of +/−D %,wherein D % represents effects of image distortions.

FIG. 7 shows an image feature 714, which could be a contiguous area ofnear-constant colored pixel that is, for instance, a human face, or partof a picture on a wall. This is detected as a blob at location (cx,cy)710 and scale (sx,sy) represented by ellipse 702. These location andscale values are used in the global descriptor generation procedurepreviously described above. FIG. 7 shows that two additional descriptorsmay be generated by first multiplying the scale by 0.9 and 1.1 togenerate the representative ellipses 706 and 704, respectively. Forthese two descriptors, the location is kept constant and the scale isperturbed by the 0.9 and 1.1 multipliers. Alternatively, the scale maybe kept constant and the location is moved by a delta amount in the xand y directions (dx,dy) to move the representative ellipse 702 to a newcenter location (cx+dx,cy+dy) 712 with representative ellipse 708. Anadditional descriptor for the representative ellipse 708 may begenerated using this new center location.

In addition, these multiple descriptors can be generated either forreference clips, the reference side, or for query clips, the query side,or both. By increasing the number of descriptors on either the referenceside or the query side the chances of detecting a match are improved, aslong as the multiple descriptors cover the errors in the featuredetection. Using multiple descriptors on both the reference side andquery side tends to further improve the chances for matching. While somemultimedia content may experience an increase in the number of falsematches with use of multiple descriptors, generally the use of multipledescriptors improves the chances of detecting matches. Using multipledescriptors on the reference side increases the size of the referencedatabase, while using multiple descriptors on the query side increasesthe query fingerprint size. While the use of multiple descriptors onboth the reference side and the query side may lower the search speed,detecting matches is generally improved. In one approach, theseincreases may be compensated through use of parallel processingtechniques.

The technique of generating multiple descriptors is applicable to otherdescriptor generation procedures, in addition to the global descriptorprocedure described above. A descriptor generation procedure that takesas input a scale-space feature with a specified location and scale,generating a compact N dimension vector for a rectangular area ofpixels, may also be used.

To enable efficient search capabilities, the descriptors are processedto generate a compact representation that is referred to as a signature.These signatures are implemented as bit vectors. In one embodiment, eachdimension of an M-dimensional descriptor generates a single bit in anoutput signature. For example, a 64-bit signature would be generated fora 64-dimensional descriptor. In other embodiments, two or more bits maybe used per dimension of the descriptor. A traversal hash signature isgenerated to facilitate a fast lookup mechanism. In one embodiment,multiple dimensions of the M-dimensional descriptor are combined togenerate a single bit in the traversal hash signature. In otherembodiments, multiple dimensions of the M-dimensional descriptor arecombined to generate multiple bits in the traversal hash signature.

The signature bits are produced by quantizing each dimension of thedescriptor independently. First, the descriptor is normalized in someway, such as made into a unit norm. For example, a unit norm vector maybe generated by dividing each component of the vector by the square rootof the sum of the squares of each component, using a technique commonlyreferred to as the L2-norm. For a one-bit-per-dimension signature, thevalue of each dimension of the normalized descriptor is compared againsta pre-specified threshold value. The resulting bits in the signaturerepresent whether the descriptor dimension is above (1) or below (0) thethreshold. In another embodiment, which may be more efficient, thedescriptor is not normalized, but rather the threshold is scaledaccording to an L2-norm of the descriptor. In yet another embodiment,the threshold may be simply set to an average value of all thecomponents of the descriptor.

FIG. 8A illustrates an exemplary signature generation process 800 inaccordance with the present invention. All positive global descriptorvalues 802 are shown as vertical bars with a threshold 804 shown as adashed line. The threshold 804 is either set according to priorempirical data or determined by the descriptor values. Descriptor valuesgreater than the threshold 804 produce a ‘1 ’ and less than or equal tothe threshold 804 produce a ‘0 ’ generating signature 806.

Choosing the threshold 804 may be done empirically. The goal is togenerate unique signatures, which are more likely unique when thenumbers of ones and zeros are approximately equal. A threshold that isset too high may generate signatures that are mostly zeros, whereas athreshold that is set too low may generate signatures that are mostlyones. The threshold should be chosen so as to avoid either extreme.

In order to facilitate fast search, a mechanism to search over only asmall part of the database is required. We enable this by generating ashorter signature, which acts as a direct hash value for the primarysignature. This shorter signature is termed a “traversal hash signature”and is associated with a set of signatures that tend to be similar toone another. The traversal hash signature may also be generated at thesame time as the primary signature. For example, for a 64-dimensionaldescriptor, in addition to a 64-bit signature, such as signature 806, anH-bit traversal hash signature 808 may be generated, where H<64. Thistraversal hash signature 808 may be based on combining the values ofvarious dimensions into a single combined value and then, similar to thesignature generation process, thresholding the combined values with atraversal hash threshold 810. For example with H=16, the values of thedimensions in groups of four are combined by either taking the sum ofthe four values, the average of the four values, or the sum of thesquare of the four values. For the global descriptor values 802, ameaningful group of four is the four values for a block which whencombined produce the combined descriptor values 812. For rectangularblocks, such as the rectangular block 506, there are 16 blocks with eachblock corresponding to one bit in the traversal hash signature 808.

A traversal hash signature may be augmented with additional bits thatrelate to an underlying feature, or perhaps to another description ofthe image data. For an underlying feature, additional bits may be addedthat identify the spatial location of the underlying feature, the scaleof the underlying feature, or the peak polarity of the underlyingfeature. For example, the spatial location of the underlying feature maybe described by two bits representing the quadrant where the center ofthe underlying feature is located. A quadrant may be indicated by havingone bit indicate top or bottom half and the other bit indicate left orright, such that the quadrant is identified with the two bits. The scaleof the feature may be quantized to either one or two bits, by settingone or more thresholds.

FIG. 8B illustrates a threshold scale 838 used in a procedure to augmenta traversal hash signature in accordance with the present invention.From the scale values sx and sy, a normalized scale value s may becomputed according to the equation: s=sqrt(sx*sx+sy*sy), represented onthe threshold scale 838 as a value on a linear scale s 820. This isevaluated against multiple thresholds. If s is between thresholds T0 822and T1 824, with threshold T0 822 inclusive, then the scale hash bitsare set to “00” 830; if s is between thresholds T1 824 and T2 826, withthreshold T1 824 inclusive, then the scale hash bits are set to “01”832; ifs is between thresholds T2 826 and T3 828, with threshold T2 826inclusive, then the scale hash bits are set to “10” 834; ifs is greaterthan or equal to threshold T3 828, then the scale hash bits are set to“11” 836. The peak polarity of the underlying feature may be indicatedby the feature detection algorithm which indicates whether a featurebased on a contrast polarity threshold is a light coloring on a darkbackground or vice-versa using one bit to represent this characteristic.

The additional bits to augment the traversal hash signature may be basedon other descriptors of image data or other data, such as texture ormotion. For example, an intensity histogram may be derived for the pixelarea 506 in FIG. 5A corresponding to the feature. The intensityhistogram may be heavily quantized to produce a small number of bits.Another approach would be to use motion analysis to indicate whether ablob is moving from frame to frame. One bit can indicate whether thereis motion or not. The approximate direction of motion may also bequantized to a small number of bits.

The overall hash traversal hash signature is a concatenation of thetraversal hash signature generated by appropriately combining andthresholding the descriptor, as illustrated in FIG. 8A, with additionalbits generated from scale, as illustrated in FIG. 8B, x/y, peakpolarity, or the like.

An advantageous feature of the present invention is the ability toselect signatures for inclusion in the fingerprint database andeliminate certain signatures that do not improve identification accuracyand lead to false positives. Various criteria, such as the distributionof ones and zeros in the primary signature and the traversal hashsignature, are used to determine whether a signature should be includedin the database. Good signature selection may reduce the size of thefingerprint database, reduce the false positive rate and improve ormaintain the overall accuracy. In a similar manner, the number ofsignatures produced for a query multimedia clip may be reduced. Asdescribed in more detail below, these advantageous features may beachieved by examining both the signatures and descriptors, individuallyand in aggregate. This examination is based on multiple criteria, suchas descriptor information, signature information, temporal robustness,and spatial diversity.

FIG. 8C illustrates a signature selection procedure 860 which selectssignatures for a particular frame of video in accordance with thepresent invention. The inputs for a frame of video are a set ofsignatures and associated traversal hash signatures 866, a set ofdescriptors 864 associated with the set of signatures 866 for the frame,a set of scalex, scaley, centerx, centery values 862 associated with theset of signatures 866; and sets of signatures and associated traversalhash signatures 868 for a previous frame. For each signature in the set866, there is a corresponding descriptor in the set 864 from which thesignature is derived, as well as a corresponding set of scale andlocation values in the set 862, from which the descriptor is derived.The sets of signatures 868 do not necessarily have any correspondence tothe signatures in the set 866. Various computations described below areperformed based on the inputs above.

For each descriptor in the set 864, descriptor information 872 iscomputed. The descriptor information 872 is associated with thecorresponding signature. Descriptor information may include a sum of thedescriptor values after normalization to unit norm. Descriptors thattend to produce false positives have energy concentrated in only a fewdimensions and hence have a lower “descriptor information” value. As anexample, this tends to happen, for instance, where a descriptor pixelregion of interest contains a bright blob on a dark background ofconstant intensity. Such regions of interest can be confused with otherregions with similar characteristics and lead to false matches. Theseregions of interest lead to descriptors with energy concentrated in afew dimensions, with those dimensions corresponding to the blob center.

For each primary signature in the set 866, signature information 874 iscomputed. The signature information 874 is associated with thecorresponding signature. Signature information may include a count ofthe number of l′s in a signature. Signatures that tend to produce falsepositives have only a few bits turned on and hence have lower “signatureinformation” value. This tends to happen for the same reasons as thedescriptor. While “signature information” and “descriptor information”are related, their thresholds can be picked independently to providemore flexibility.

For each signature in the set 866, a temporal robustness value 876 iscomputed, using the sets of signatures 868. The computed temporalrobustness value 876 is associated with the corresponding signature inthe set 866. Temporal robustness may include a measure of whether afeature appears in a following frame. The frame lag between a frameunder consideration and the following frame may be one frame, theimmediately following frame, or more than one frame. An estimate isgenerated to indicate whether a feature appears in a following framebased solely on the signature and traversal hash signature. FIG. 8Dillustrates a procedure 881 to generate the temporal robustness estimatein accordance with the present invention. A set of signatures andtraversal hash signatures for the current frame 882, is compared againstall signatures and traversal hash signatures from following frame 883 bycomputing the bit distance between the respective signatures intraversal hash signature comparison computation 884 and the bit distancebetween the respective signatures in signature comparison computation885. The bit distances are computed for each signature in 882, and thebit distances are compared against a robustness estimate thresholdcomputation 886 to produces a set of temporal robustness values 887, onevalue corresponding to each signature in 882. Specifically, a signatureis considered “temporally robust” if there exists at least one pair ofsignature and traversal hash signature found in the traversal hashsignature comparison computation 884 and in the signature comparisoncomputation 885 with a signature bit distance≦R and a traversalsignature bit distance≦Q. The values R and Q are integer-valuedparameters that can be specified. In one embodiment, a temporalrobustness value is either 0 or 1, based on whether the signature is“temporally robust”.

For each feature scale and location value in the set 862 in FIG. 8C, aspatial diversity value 870 is computed by comparing its own valueagainst the entire list of feature scale and location values in the set862. The computed spatial diversity value 870 is associated with thecorresponding signature. Spatial diversity may include a measure ofwhether a feature is located close to other features. For example, thedistance between any pair of feature locations can be computed for theset 862, and used to compute the average distance between a featurecenter location and the M near features' center locations, where M maybe pre-specified. This average distance, or “closeness” indicates thedensity of features in terms of location. Ideally, the features would bespread out and those features that are concentrated too close togetherwould be penalized. By incorporating this “closeness” criterion,features that are spread out are preferred, according to a proceduredescribed in more detail below.

Each signature in the set 866 is now associated with the results of thecomputations in blocks 870, 872, 874, and 876. The results are input tothe signature sorting and selection block 878. Signatures that fail tomeet some minimum level of criteria are eliminated. For descriptorinformation, a minimum threshold may be used. Likewise, for signatureinformation, another minimum threshold may be used. For temporalrobustness, Q and R bit distance parameters may be set, and signaturesthat are not considered temporally robust according to this criterionare eliminated. Similarly, for spatial diversity, a threshold can beestablished for “closeness”, and signatures that are too close based onthis threshold are eliminated.

Signatures may also be sorted, for example, according to the descriptorinformation, the signature information and the spatial diversity,descending, in that order. The sorting provides a list of signatures inapproximate order of strength. Then, all but the T top signatures areeliminated, where T is a parameter that can be pre-specified. The resultis a set of signatures 880 that satisfy the minimum criteria and/or arethe top T signature according to some criteria.

The selected signatures including the signature and traversal hashsignature pairs are stored along with associated metadata such asmultimedia id, playout timestamp, and scale, and x and y location valuesto create a signature record that is inserted into the referencesignature database. The signature database consists of the set of allsignature records for all corresponding reference multimedia along withtwo indices that are used for different types of access of the signaturerecords. These indices are a hash signature index and an index based onthe multimedia id and playout timestamp. There may be a single set ofsignature records pointed to by both indices, or two sets of signaturerecords, with identical data, for example, one for each index.

FIG. 9A illustrates an exemplary signature database 900 organized by aprimary hash signature index in accordance with the present invention.The signature records for all the multimedia content that is to be putinto the database is collected together and grouped by the traversalhash signature. At this stage of processing, the number of signaturesthat belong to particular traversal hash signature is known so thememory for the signature records can be allocated and signature recordsmay be stored in memory. It is advantageous for search operations thatthe signature records for a particular traversal hash signature bestored contiguously. The set of signatures belonging to a traversal hashsignature is called a cluster. For example, 100 signatures andcorresponding traversal hash signatures may be generated having 50signatures with a traversal hash signature A, 30 signatures with atraversal hash signature B and 20 signatures with a traversal hashsignature C. Thus, the 100 signatures are organized into three clusters,cluster A with 50 signatures, cluster B with 30 signatures and cluster Cwith 20 signatures, that are stored in memory.

For each traversal hash signature, the number of signatures and apointer to the location where the corresponding signature records beginis stored for processing. Since the space of traversal hash signaturesmay be relatively small, for example, a 16-bit traversal hash signatureimplies a maximum 65,536 entries, all possible traversal hash signaturescan be indexed in an array. A traversal hash signature array 902 isshown in FIG. 9A storing one element for each possible traversal hashsignature. The index into the traversal hash signature array 902 is theinteger interpretation of the traversal hash signature as a binarynumber. Thus, given a traversal hash signature, direct addressing intothe array retrieves the number of matching signatures and wherecorresponding signature records are located, such as a link reference toa list of signature records 904 or 906. In FIG. 9A, for example,traversal hash signature “0101001010101010” is located at entry 908,which links to the array of signature records 904, and traversal hashsignature “1011010100101010” is located at entry 910, which links to thearray of signature records 906. Each entry in traversal hash signaturearray 902, such as entries 908 and 910, have an additional fieldincluded in the entry that stores the link reference address (LRA) to asignature record array, such as signature record arrays 904 and 906.

FIG. 9B illustrates an exemplary signature database 950 organized bysecondary index in accordance with the present invention. The secondaryindex supports accesses of the signature records corresponding to amultimedia id, from a starting playout time to an ending playout time.The secondary index is useful for the signature correlation process,discussed in more detail below. The secondary index is based on a hashtable 952 organized by multimedia id. For example, a hash table entry958 corresponding to multimedia id “vo102340910” points to a linked list960. Each entry in the linked list, for example, 962 and 964, points toan array of signature records, for example, 954 and 956. The signaturerecords are stored in playout timestamp order within each array, and thelinked list is organized with arrays in playout timestamp order.Alternatively, instead of 954 and 956 representing an array of signaturerecords, they may represent an array of pointers to signature records,already stored according to the primary index. Hence, the secondaryindex does not store the signature records again, but rather points tothe signature records of the first index. This is the case where bothindices shared a common set of signature records, as mentioned above.Such an alternative approach can be more space efficient, but may beslower to access due to signatures that are randomly distributedrelative to the second index.

The secondary index allows for searches such as ‘all referencesignatures for multimedia id “vol 02340910” between time period 817000milliseconds and 1048500 milliseconds’. In this case, the entry 958corresponding to vo102340910″ is first accessed, retrieving the linkedlist 960, starting with the linked list entry 962. The array ofsignatures 954 can be examined as to whether the time period 817000 to1048500 milliseconds is contained. Any matching signatures are returned.The subsequent entries on the linked list, for instance 964, areprocessed, until there are no more entries, to ensure that all matchingsignatures within the time period are retrieved. This search can be madefaster by keeping the starting and end timestamp of an array ofsignatures within the corresponding linked list entry itself, so thatthe signatures themselves do not need to be accessed unnecessarily.Also, the requested signature record for the desired starting timestampis found by binary search through the array. From there, signaturerecords in the array are processed linearly until the desired endingtimestamp is reached.

The video database structure may be stored either in the localcomputer's main memory (its RAM) or on a hard disk drive. One embodimentis to store the signature database in main memory as access speeds aresignificantly faster. The trade-off is the smaller capacity of mainmemory versus the hard drive.

A query multimedia clip is processed to generate signatures as describedabove and using the process of similarity search against a referencedatabase, the most likely candidate clips may be identified. Thebeginning step in a similarity search is to find all near-matchingsignature records in the video database for a query multimedia clip. Thesimilarity search process begins with generating signature and traversalhash signature pairs for the query multimedia clip. A playout timestampis also associated with each signature and traversal hash signaturepair. The traversal hash signature is used to dramatically speed up thisportion of the search. The result of this initial search is a list ofmatching signatures and their associated metadata called the candidatelist. In a following step, the number of times a multimedia clip appearsin the candidate list is counted to generate a frequency list ofmultimedia clip identifications (ids), which is then sorted from mostfrequent to least frequent. The most frequent clip ids on this list arethe likely matching multimedia clips. In a correlation step, the topmatching multimedia clips are analyzed to find the closest matchingclip. In the correlation step, signatures of the top matching clips areused instead of matching traversal hash signatures, and a score iscomputed based on the amount of signature bit error. This score iscompared against prespecified thresholds to decide whether thecorresponding multimedia clip is a good match to the query clip. Thus,one or more closest matching multimedia clips from the referencedatabase may be chosen for presentation as a result of the search.

FIG. 10 illustrates an exemplary similarity search process 1000 inaccordance with the present invention. For each signature and traversalhash signature pair 1002 in a list of query signatures 1004, asimilarity search is performed given the parameters of signaturedistance R and traversal hash signature distance Q. A traversal hashsignature distance of Q=0 means that only an exact match of thetraversal hash signature is allowed, thus only a single cluster needs tobe searched, which is one of the embodiments of the present invention. Atraversal hash signature distance of Q=1 means that traversal hashsignatures which are a maximum of 1 bit distance from the specifiedtraversal hash signature are searched. For an K-bit hash signature, thiswould mean that a total of K+1 clusters need to be searched includingthe original cluster, as well as the K clusters that correspond toexactly one of the bits flipped and all other bits unchanged. The numberof clusters needed to be searched increases exponentially with Q.

In the case of Q=0, the traversal hash signature “0101010100101010” 1003in the signature and traversal hash signature pair 1002 obtained fromthe list of query signatures 1004 is used to look up entry 1012 in thetraversal hash signature array 1006 and obtain a set of correspondingsignature records 1008. This set contains the potentially matchingsignatures with the same traversal hash signature. The query signature1009, shown as 31 bits of a 64 bit query signature“1011101001001010100010101010101 . . . ” in the pair 1002 is nowcompared with each signature in the set 1008. Specifically, the distancebetween the query signature and each signature in the set to becompared, is computed. This computation may be made more efficient byusing bit operands and a bit-count lookup table. The Hamming or bitdistance between two 64-bit numbers can be computed by performing an XORon the two numbers and then count the number of 1's in the result. Thecounting step may be implemented with a lookup table that maps an 8-bitor 16-bit number to the number of l′s contained. The number of one's inthe result of the XOR is then the sum of the lookup table results forall of the 8-bit or 16-bit chunks contained in that XOR result. If thedistance computed is less than or equal to the specified signaturedistance R, then this signature record is inserted into a candidate list1010.

An entry in the candidate list 1010 consists of a query clip timestamp,a matching database clip multimedia id, a matching database cliptimestamp, and a signature bit distance. For each signature andtraversal hash signature pair, several entries may be added to thecandidate list 1010. Similarity search is repeated for each signatureand traversal hash signature pair 1002 in the list of query signaturerecords 1004 corresponding to the query multimedia clip. Depending onthe length of the query clip, the candidate list 1010 may have hundredsor thousands of entries.

FIG. 11 illustrates a query multimedia clip partitioned into smallersegments 1100 in accordance with the present invention. The querymultimedia clip 1102 may additionally be broken into smaller overlappingsegments 1104-1106, and similarity search and a top matches andcorrelation procedure performed on the smaller segments. Thispartitioning allows more time granularity in the multimedia clipidentification, as well as greater accuracy in identifying matchingmultimedia clips. In one embodiment, the entire query multimedia clip isbroken into small clip segments that overlap by as much as 50% with theneighboring clip segments.

FIG. 12 illustrates an exemplary listing of top matches 1200 inaccordance with the present invention. The candidate list 1202 thatcorresponds to a query multimedia clip, or some small segment of it, maybe processed to produce the most likely matching multimedia clips. In afirst step, the number of times a database multimedia clip appears inthe candidate list is counted, to generate a frequency list ofmultimedia clip id's and the frequency of that clip's appearance in thecandidate list. In a second step, the frequency list is then sorted indescending order of frequency to generate a sorted frequency list 1204representing the top matches.

For application scenarios that require low computational burden, forinstance where a search needs to be extremely fast, or where the queryclips are known to have little or no image distortion, due to the natureof the application, the correlation step can be omitted. In this case,the most frequent database multimedia clip, which would be representedby the first multimedia clip id in the sorted frequency list 1204, maybe simply output as the identity of the unknown query clip.Alternatively, for other scenarios, by taking the top V most frequentmultimedia clips, the first V multimedia clip id's in the sortedfrequency list 1204, the candidate list has been focused in on the mostlikely matches. Next, signature correlation processing is used toeliminate potential false matches and ensure, with greater accuracy,that a correct matching multimedia clip has been found.

FIG. 13 illustrates a signature correlation process 1300 in accordancewith the present invention. Further information regarding a procedurefor using signature correlation to eliminate false positives and gettime-accurate matches may be found in U.S. Provisional Application Ser.No. 61/181,806 filed on May 28, 2009 entitled “Multi-Media ContentIdentification Using Multi-Level Content Signature Correlation and FastSimilarity Search”, FIGS. 1A, 2A, 2B, 2C, 3A, 3B, 4-6, pages 11-26. Thesignal correlation process 1300, in the context of a signature database,is described next. One of the goals of the signature correlation process1300 is to verify that the signatures of a potentially matchingreference multimedia clip 1304 do indeed correspond with the signaturesof the query multimedia clip 1305.

In a first step 1302, a time range of the potentially matching referencemultimedia clip is determined from the reference database. For example,if the potentially matching reference multimedia clip in the referencedatabase is subdivided into smaller clips, then an approximate timerange may be determined. Otherwise, and potentially in addition, thematching time range can be inferred from the candidate list 1306 byanalyzing a correspondence in time between the database clip and thequery clip, using a Hough transform.

In a second step 1308, with a matching time range and the associateddatabase clip, corresponding signatures are accessed from the signaturedatabase using the secondary index as illustrated in FIG. 9B. Generally,the corresponding signatures are a small set of signatures. Since onlythe matching time range is accessed, the size of the set of signaturesremains small, even as the size of the database grows, allowing thecorrelation step to scale with the database size. The signatures for thematching time range of the reference are now compared with the querysignatures. For each query signature, a similarity search is performed,with a specified signature distance, within this small set of signaturesfrom the reference database. This step is used to determine the bitdistance from each query signature to each reference signature, and isused in the next step to determine scores.

In a third step 1310, for each query signature, a score is derived basedon the distances to the closest signatures in the reference databasesignature subset, and the number of matches. For instance, the scoreassigned to a query signature may be the minimum bit distance to all thesignatures in the reference signature set, with a particular maximumallowable value. The score for each query signature may be averaged forthe entire set of query signatures to give an overall score for theparticular reference clip match. According to the steps above, a lowscore implies low bit distance between the query and reference, andhence a good match.

In a fourth step 1312, the overall score is subjected to a thresholdtest which determines whether the database clip is considered a match1314. This threshold must be appropriately selected, based onapplication requirements, to trade-off between a high match rate and alow false positive rate.

Those of skill in the art will appreciate that based on the presentdisclosure additional alternative systems and methods for scalable,accurate and distortion robust media fingerprinting and identificationof digital video sequences may be determined in accordance with thedisclosed principles of the present invention. Thus, while particularembodiments and applications of the present invention have beenillustrated and described, it is to be understood that the invention isnot limited to the precise construction and components disclosed hereinand that various modifications, changes and variations which will beapparent to those of ordinary skill in the art may be made in thearrangement, operation and details of the method and apparatus of thepresent invention disclosed herein without departing from the spirit andscope of the invention as defined in the appended claims. For example,in another embodiment, an alternate method of feature detection otherthan the two-pass scale space approach, is used. A feature detectionmethod that provides the center location and spatial extent of thefeature, which may be described as a “scale space” method, is feasible.In yet another embodiment, an alternate method for generating adescriptor is used other than the “global descriptor” method presentlydescribed. The “phase descriptor” method is one such alternative, and itis described in U.S. application Ser. No. 12/612,729 filed Nov. 5, 2009entitled “Digital Video Content Fingerprinting Based on Scale InvariantInterest Region Detection with an Array of Anisotropic Filters”, FIGS.9, 10, and 11 and pages 26-28, 38, and 39. Other approaches thatgenerate a compact N dimension vector for a rectangular area of pixelsare applicable.

1. A method of searching reference multimedia signatures that representaudio and video content of reference multimedia clips for acorresponding query multimedia clip, the method comprising: generatingan N dimension signature based on an N dimension descriptor and atraversal hash signature based on a combination of dimensions of the Ndimension descriptor for each feature identified in a set of referencemultimedia clips; storing each N dimension signature and a link to acorresponding reference multimedia clip at a location in a referencesignature database addressable by the traversal hash simature;generating an N dimension query signature based on an N dimension querydescriptor and a traversal hash query signature based on a combinationof dimensions of the N dimension query descriptor for a query multimediaclip; and searching the reference signature database using the traversalhash query signature to find similar signatures that are within aspecified signature distance, wherein the similar signatures areaggregated in a candidate list of closely matching signatures thatcorrespond to similar reference multimedia clips.
 2. The method of claim1, wherein a multimedia clip comprises image pixel data, video pixeldata, or audio waveform data.
 3. The method of claim 1, furthercomprising: generating an N dimension descriptor for at least onefeature of a reference multimedia clip; and quantizing each dimension ofthe N dimension descriptor to generate the N dimension signature.
 4. Themethod of claim 3, further comprising: creating a Bx by By grid havingBx*By blocks located at the center of the at least one feature, the atleast one feature having dimensions specified by a scalex and a scaleymultiplication factors, each block having a Cx by Cy pixel grid;computing multiple intensity differences for each pixel in the Cx by Cypixel grid to another pixel in both x and y directions for a specifiedpixel offset that is greater than 1; computing weighted sum combinationsof the multiple intensity differences as x-gradient values and as ay-gradient values; and generating the N dimension descriptor as aconcatenation of combinations of the x-gradient values and y-gradientvalues for each block of the Bx*By blocks.
 5. The method of claim 4,wherein the combinations of the x-gradient values and y-gradient valuesare combinations of a positive sum of x-gradient value, a negative sumof x-gradient values, a positive sum of y-gradient values, and anegative sum of y-gradient values for each of the Bx*By blocks, whereinthe N dimension descriptor is an N=4*Bx*By dimension descriptor.
 6. Themethod of claim 4, wherein the scalex multiplication factor or thescaley multiplication factor or both are adjusted to account for anexpected error margin of +/−D %, wherein D % represents effects of imagedistortions.
 7. The method of claim 4, wherein the location of thecenter of the feature is adjusted to account for an expected errormargin of +/−D %, wherein D represents effects of image distortions. 8.The method of claim 1, further comprising: combining multiple dimensionsof the N dimension descriptor to form combined values; and comparing thecombined values with a traversal hash threshold to form the traversalhash signature.
 9. The method of claim 8, further comprising: combiningmultiple dimensions of the N dimension query descriptor to form querycombined values; and comparing the query combined values with thetraversal hash threshold to form the traversal hash query signature. 10.The method of claim 1, wherein the traversal hash signature includesadditional bits representing a quadrant having the center of anunderlying feature and a scale of the underlying feature.
 11. The methodof claim 1, wherein the traversal hash signature includes a polarityvalue that indicates whether the underlying feature is a light coloringon a dark background or a dark coloring on a light background.
 12. Themethod of claim 1, further comprising: sorting the candidate listaccording to a count of the number of times a closely matching signatureappears in the candidate list to find likely reference multimedia clipmatch signatures.
 13. The method of claim 12, further comprising:determining a plurality of scores, wherein each score is based on adistance between the N dimension query signature and a different one ofthe likely reference multimedia clip match signatures; and comparing theplurality of scores with a score threshold to determine whether areference multimedia clip associated with a score is a true positivematch or a false positive match.
 14. The method of claim 1, furthercomprising: counting a frequency of occurrence of multimedia clipidentifications associated with each closely matching signature in thecandidate list; and sorting by the frequency of occurrence to find Tlikely reference multimedia clip matches, where T=1 or T>1.
 15. Themethod of claim 1, further comprises: generating features from themultimedia clips; computing one or more descriptors for each of thegenerated features; and computing one or more signatures and associatedtraversal hash signatures for each of the one or more descriptors. 16.The method of claim 1, further comprising: storing a secondary copy ofeach N dimension signature for each corresponding reference multimediaclip according to a multimedia id and timestamp as a secondary index,wherein the traversal hash signature is a primary index.
 17. The methodof claim 16, further comprises, comparing the N dimension querysignature to signatures in the reference signature database associatedwith the traversal hash signature to find similar signatures that arewithin a specified signature distance.
 18. A method of forming accuratedescriptors of features in multimedia clips, the method comprising:determining a scale and a location of the center parameters for eachfeature of a plurality of features selected from a frame from amultimedia clip; creating a Bx by By grid having Bx*By blocks located atthe center of each feature and each block having a Cx by Cy pixel grid;computing for each block, functions of multiple intensity differencesbetween each pixel in a block's Cx by Cy pixel grid and another pixelfor a specified pixel offset that is greater than 1 and based on thescale and location of center parameters; generating a first N dimensiondescriptor as a concatenation of the functions for each block of theBx*By blocks; and adjusting individually the scale and location ofcenter parameters to account for an expected error margin due to imagedistortions to generate at least a second N dimension descriptor as aconcatenation of the functions for each block of the Bx*By blocks asadjusted according to the expected error margin.
 19. The method of claim18, wherein the scale is specified by a scalex and a scaleymultiplication factors and the multiple intensity differences arecomputed in both x and y directions based on the scalex, scaley, andlocation of center parameters.
 20. The method of claim 19, wherein thefunctions of the multiple intensity differences are combinations of apositive sum of x-gradient value, a negative sum of x-gradient values, apositive sum of y-gradient values, and a negative sum of y-gradientvalues for each of the Bx*By blocks, wherein the x-gradient values areweighted sum of combinations of the multiple intensity differences inthe x direction and the y-gradient values are weighted sum ofcombinations of the multiple intensity differences in the y directionand N is equal to 4*Bx*By.
 21. A computer readable medium storing acomputer program which causes a computer system to perform a method ofsearching reference multimedia signatures that represent audio and videocontent of reference multimedia clips for a corresponding querymultimedia clip, the method comprising: generating an N dimensionsignature based on an N dimension descriptor and a traversal hashsignature based on a combination of dimensions of the N dimensiondescriptor for each feature identified in a set of reference multimediaclips; storing each N dimension signature and a link to a correspondingreference multimedia clip at a location in a reference signaturedatabase addressable by the traversal hash signature; generating an Ndimension query signature based on an N dimension query descriptor and atraversal hash query signature based on a combination of dimensions ofthe N dimension query descriptor for a query multimedia clip; andsearching the reference signature database using the traversal hashquery signature to find similar signatures that are within a specifiedsignature distance, wherein the similar signatures are aggregated in acandidate list of closely matching signatures that correspond to similarreference multimedia clips.
 22. The computer readable medium of claim21, further comprising: generating an N dimension descriptor for atleast one feature of a reference multimedia clip; creating a Bx by Bygrid having Bx*By blocks located at the center of the at least onefeature, the at least one feature having dimensions specified by ascalex and a scaley multiplication factors, each block having a Cx by Cypixel grid; computing multiple intensity differences for each pixel inthe Cx by Cy pixel grid to another pixel in both x and y directions fora specified pixel offset that is greater than 1; computing weighted sumcombinations of the multiple intensity differences as x-gradient valuesand as a y-gradient values; generating the N dimension descriptor as aconcatenation of combinations of the x-gradient values and y-gradientvalues for each block of the Bx*By blocks; and quantizing each dimensionof the N dimension descriptor to generate the N dimension signature. 23.The computer readable medium of claim 22, wherein the combinations ofthe x-gradient values and y-gradient values are combinations of apositive sum of x-gradient value, a negative sum of x-gradient values, apositive sum of y-gradient values, and a negative sum of y-gradientvalues for each of the Bx*By blocks, wherein the N dimension descriptoris an N=4*Bx*By dimension descriptor.
 24. The computer readable mediumof claim 22, wherein the scalex multiplication factor or the scaleymultiplication factor or both are adjusted to account for an expectederror margin of +/−D %, wherein D % represents effects of imagedistortions.
 25. The computer readable medium of claim 22, wherein thelocation of the center of the feature is adjusted to account for anexpected error margin of +/−D %, wherein D % represents effects of imagedistortions.
 26. The computer readable medium of claim 21, furthercomprising: storing a secondary copy of each N dimension signature foreach corresponding reference multimedia clip according to a multimediaid and timestamp as a secondary index, wherein the traversal hashsignature is a primary index.
 27. The computer readable medium of claim26, further comprises: comparing the N dimension query signature tosignatures in the reference signature database associated with thetraversal hash signature to find similar signatures that are within aspecified signature distance.
 28. A method for creating a mediadatabase, the method comprising: generating a plurality of signaturesfrom descriptors of selected media content; selecting a signature fromthe plurality of signatures based on a distribution of bit values in theselected signature and on values of a corresponding descriptor;generating a traversal hash signature from the corresponding descriptor;and storing the selected signature in a media database at a locationwithin a set of contiguous locations based on the traversal hashsignature used as a primary index.
 29. The method of claim 28, furthercomprising: storing a second copy of the selected signature in the mediadatabase at another location within a set of contiguous locations basedon a multimedia identification value and a timestamp of the mediacontent associated with the selected signature.
 30. The method of claim28, further comprising: selecting a signature from the plurality ofsignatures that are associated with a corresponding descriptor made upof descriptor values and identified by a function of the descriptorvalues that meets or exceeds a threshold value.
 31. The method of claim28, further comprising: computing an average distance between a featurecenter location and M selected near features' center locations todetermine a spatial diversity value, wherein the average distance, or“closeness” indicates the density of features in terms of location; andeliminating signatures that are too close based on an establishedthreshold for closeness.
 32. The method of claim 28, further comprising:computing signature bit distance values between each signature for acurrent frame in the media database and each signature in a followingframe in the media database; determining whether the signature bitdistance values are less than or equal to a first robustness thresholdR; and eliminating signatures that have bit distance values greater thanR that are not temporally robust.
 33. The method of claim 28, furthercomprising: computing hash bit distance values between each traversalhash signature for a current frame in the media database and eachtraversal hash signature in a following frame in the media database;determining whether the hash bit distance values are less than or equalto a second robustness threshold Q; and eliminating traversal hashsignatures that have bit distance values greater than Q that are nottemporally robust.